1. Field of the Invention
This invention relates generally to a heat exchanger, and more particularly to a shell-and-tube heat exchanger including triple-wall heat-conducting tubes.
2. Description of the Prior Art
A conventional fast breeder reactor plant includes steam generators, where water in metal tubes is heated and boiled by heat transferred from hot liquid metal outside of the tubes in a shell. The liquid metal gains heat in a reactor vessel, while the steam produced in the steam generators is used to generate electric power in turbine generators.
If the tube wall segregating the water from the liquid metal has a defect and the water leaks into the liquid metal, and if the liquid metal is sodium, as usually is the case, the liquid metal and the water will react explosively. In order to avoid such an incident, shell-and-tube heat exchangers using double-wall tubes have been developed, as shown in pages 60 through 87, pages 270 through 279 and pages 280 through 288, of Nuclear Technology Vol. 55, Nov. 1981.
In the double-wall tube steam generators disclosed in the above references, high pressure water flows in inner tubes, and sodium flows outside of outer tubes in a shell. There are small gaps between the inner tubes and the outer tubes, and the gaps are connected to a gas plenum or a monitoring chamber. If there is a defect in the inner tube, the water or steam flows into the gap and then into the gas plenum, which can be detected. If there is a defect in the outer tube, the gas in the gas plenum flows into the sodium in the shell, and the gas plenum pressure decreases, which can be detected.
The gaps should be large enough for the leakage particles to diffuse rapidly in order to induce a rapid response by the detector. However, since the gaps hinder heat transfer, the double-wall tubes with gaps require a large heat transfer area, which results in large and expensive heat exchangers. Besides, the inner tubes and the outer tubes must be metallurgically separated in order to avoid cracks in the inner tubes expanding into the outer tubes, or vice versa. Furthermore, the size of the gaps cannot be controlled, because they change due to heat expansions of the inner and outer tubes.
The outer tubes disclosed in the above-mentioned references have grooves in the axial direction on the inner surface, to promote diffusion of the leakage particles. However, the outer tube thickness must be increased due to the grooves, which increases the heat resistance and also the cost of the tubes.
A heat exchanger including triple-wall tubes with porous intermediate layers is disclosed in German Patent Publication AUSLEGESCHRIFT No. 1117148. However, it is not easy to construct such a heat exchanger with the porous intermediate layers in good thermal contact with the inner and outer tubes.